The present invention relates to a process for modifying the section of a railway rail comprising a flange, a web and a head, consisting in increasing the height of the rail to the detriment of the thickness of the web and of the flange.
It is well known (see, for example, French Patent No. 751,242) to modify the section of a rail so as to adapt it to that of a rail of different section in order to be able to be welded or attached thereto. This is the case, for example, in points where the heels of the points not only have a greater section, but also a different (for example asymmetrical) shape from that of the rails of the normal track. In order to be able to join the ends of these rails of different sections, the profile of the rail with the greater section is generally deformed over a certain length in order to adapt it to the shape of the adjacent rail. This deformation is carried out by forging, that is to say that the rail is heated to red heat and deformed between the suitably shaped jaws of a press. If there remains an excess of material for the profile desired, it is generally contrived to upset this excess in the flange and to remove it, after forging, by a machining process.
All rails which have undergone such a forging exhibit, somewhere in the transition zone, a section having a reduced hardness which arises from the temperature gradient between that part of the rail which is heated to red heat with a view to forging it and the rest of the rail which is at ambient temperature. This reduced-hardness section results, in the long term, in a slight sag of the running surface in the head of the rail, caused by the repeated passage of the wheels.
Heating the rail to its forging temperature furthermore negatively affects the mechanical properties, especially its hardness over the entire length of the heating. This is all the more serious if the rail has undergone, as is generally the case, a prior heat treatment since the beneficial effects of this treatment are then irremediably lost in this zone.
However, given that these anomalies are relatively minor and have but little influence on the behavior of railway convoys running at low or medium speed, they have, up to now, been of hardly any concern.
On the other hand, with the appearance of high-speed trains, the requirements with respect to the quality of the rails become increasingly strict. However, the sole known means of preventing the appearance of these critical reduced-hardness zones is to prevent the thermal gradients, that is to say to heat the rail to red heat over its entire length in order to forge its end, whereas the sole known means of avoiding these critical zones and of re-establishing its initial hardness is to make the rail undergo, after forging, a heat treatment over its entire length. It goes without saying that one or other of these involves enormous means which considerably burden the manufacturing cost of such a rail.